PREPARATION OF NUTRIENT MEDIA. 33 



Amongst fluid media we need only consider milk and 

 bouillon. The latter is most successfully prepared in 

 the following way : — Half a kilogramme of pounded or 

 minced beef, as free as possible from fat, is well stirred 

 up in a litre of water. The mixture is allowed to 

 stand for from twelve to twenty-four hours (if the 

 weather is warm it is placed on ice). It is then 

 squeezed in a strong gauze bag, until about one litre 

 of liquid is pressed through. Ten grains of dry 

 peptone and five grains of common salt are added to 

 this reddish fluid, which is neutralised by being boiled 

 in a large beaker in the steam steriliser with a 

 saturated solution of chemically pure soda (sodium 

 carbonate) ; or better still, the fluid is made slightly 

 alkaline, and is then boiled for an hour in the steam 

 steriliser. By this means all the albumen, which coag- 

 ulates at boiling point, is separated out, and a clear 

 light yellow fluid, which no longer becomes turbid on 

 being boiled, is obtained by filtering.^ This filtering 

 must take place whilst the fluid is hot ; if it should, 

 however, happen that the flltrate is not quite clear, it 

 must be allowed to cool ; the white of a new-laid egg 



' If the glass vessels, in which the bouillon is later on 

 sterilised, have not heen used before, and especially if they 

 have not been boiled, the liquid is often rendered turbid, 

 because the glass has an alkaline reaction. It is therefore 

 necessary to boil all vessels before using them, if faultless 

 preparations are desired. 



